The cell walls of fungi evoke a powerful immuno-stimulatory response, and have been proposed for use as potential anti-infective and anti-tumor drugs. Fungal cells can also activate dendritic cells and prime class II restricted antigen specific T cell responses. The majority of the cell wall (50-60%) of pathogenic (Candida albicaus) and non-pathogenic fungi (Saccharomyces cerevisiae) is composed of an inner layer of β-glucan β-1,3- and β-1,6-glucan) covalently linked to a variety of cell surface mannoproteins [Klis, F. M. et al. Med Mycol 39 Suppl 1, 1-8, 2001; Klis, F. M. et al., FEMS Microbiol Rev 26, 239-56, 2002].
Recognition of β-glucans by macrophages is carried out mainly through Dectin-1 with cooperation of TLRs, including TLR2 [Brown, G. D. et al. Nature 413, 36-7, 2001]. Dectin-1 activity is inhibited by β-1,3-glucans and β-1,6-glucans, with the β-1,3-glucan laminarins having the highest effect. However, oligosaccharide microarray results show that Dectin-1 binds specifically to β-1,3-glucans. Neutrophils are professional killers, whose role in phagocytosis and killing of bacteria and fungi is well characterized. Neutropenic individuals are much more susceptible to bacterial and fungal infections, with return to normal counts playing an important role in resolution of infection. Neutrophils, unlike macrophages, require serum for optimal phagocytosis and killing. The main opsonic receptors are the complement receptor CR3 and the immunoglobulin-binding receptor FcγR. CR3 has a lectin domain [Brown, G. D. et al. Immunity 19, 311-5, 2003] that mediates increased neutrophil motility towards a mixture of β-1,3-glucan and β-1,6-glucan (PGG-glucan) [Wakshull, E. et al. Immunopharmacology 41, 89-107, 1999].
β-1,6-glucans have been found to provide potent anti-fungal activity, and inter alia, possess adjuvant activity and activate complement.
The complement (C) system of humans and other mammals involves more than 20 components that participate in an orderly sequence of reactions resulting in complement activation. Products derived from the activation of C components include non-self recognition molecules C3b, C4b and C5b, as well as the anaphylatoxins C3a, C4a and C5a that influence a variety of cellular immune responses (Hugh et al (1982) 15th International Leucocyte Culture Conference, Asilomar, Calif. (Abstract); Fujii et al. (1993) Protein Science 2:1301-1312; Morgan et al. (1982) J. Exp. Med. 155:1412-1426; Morgan (1993) Complement Today 1:56-75; Morgan et al. (1983) J. Immunol. 130:1257-1261). Complement activation occurs primarily via the “classical” pathway or the “alternative” pathway. The classical pathway is initiated by the binding of the first complement component (C1) to immune complexes through C1q, a subcomponent involved in binding to antibody. The c1 complex is composed of C1q and two homologous serine proteases, C1r and C1s (1:2:2 molar ratio). After binding to the immune complexes C1q undergoes a conformational change resulting in the conversions of C1r and C1s to their activated forms. Activated C1s cleaves C4 and C2 to generate a complex of their fragments C4b2a, which in turn cleaves C3 into C3a and C3b. C3b binds to immune complexes.
The alternative pathway is activated without involvement of antibody. C3b molecules generated from C3 by interaction of C3 with two serine proteases, factors B and D, are deposited on the microbial surface where activation of C3 is amplified. C3b produced by activation of either pathway acts as a central molecule in the subsequent formation of membrane attack complexes that can lyse microbes and also as an opsonin.
It is unknown whether β-1,6-glucans produce a robust immune response in all subjects and by what mechanism such response is generated.